Chain/ derailleur issue.

[LarryDuff]

This >>>>. Could this be a short cage derailleur with not enough capacity for such a large cassette?

I don't think so, the cassette has always been an 11-32

 

[faster]

Per Shimano, a long cage derailleur is needed for an 11-32. That doesn't look like a long cage derailleur, although I could be wrong. You'll need to check the part numbers

Maybe it would work (just!) with a shorter chain, but I reckon it would be pretty sketchy and on the verge of ripping the derailleur off the frame.

 

[Ajax Bay]

Could this be a short cage derailleur with not enough capacity for such a large cassette? The model numbers on it will confirm.

Yup! I reckon that's exactly the scenario. And sonny Jim has shoved on cassette with a dinner plate large sprocket on the direct drive trainier - think a 32t.
50-34 and 11-32 = wrap capacity needed of 37t: well beyond a 105 SS RD (model number on underside of RD parallelogram).
The chain needs to be that long (it's only just long enough as is). Just put up with the chain fouling per OP when in small ring and the smaller sprockets: it'll make a noise saying "shift up to the large chainring please". Or fit a more sensible cassette (eg 11-28) and MTFU on any scenic roads. Or replace the RD.

 

[Sharky]

Observations...

The photos show the bike on a wheel off trainer. When the gears were serviced, I presume it was on his complete bike? Does he get the same symptons on the bike, with the wheel on? Was the service done by a professional bike mechanic? If the latter, he should go back to the mechanic.

If the bike runs smoothly, then there must be something different between the bike and the trainer set up.

 

[GuyBoden]

As we know using small-small, you get chainline problems and prone to chain rub, resulting in inefficient power transfer.

But, they are still frequently used.